New Grinder Concept for Continuous Pressure Grinding, 1999 International Mechanical Pulping Conference Proceedings

Pekka Haikkala, Heikki Liimatainen & Olli Tuovinen

Valmet Oyj

Today some 100 two-pocket Valmet pressure grinders as well as 150 two-pocket atmospheric Tampella grinders are in operation worldwide making pulps for several paper and board grades from different wood species. However, as described in this paper, it has been noticed in several mill-scale studies that, due to the discontinuous or batchwise operation principle of these two-pocket grinders, there are some drawbacks, which can be eliminated or improved by introducing a new continuous grinding principle for high capacity grinding.

During this development work, the first aspect to be studied was the behavior of the logs in the grinder pockets under loading conditions as well as the short-term variations during each wood batch. The behavior of the individual logs under various loads and temperatures was then studied to learn how the logs are heated up and compressed in the two-pocket pressure grinders. These trials showed that the logs and also the wood batches are heavily compressed at the end of the main cylinder stroke, which causes deviations both in PGW pulp quality and in grinder production during each wood batch.

Several options for continuous grinder design were innovated and evaluated. Based on these ideas, log-loading pilot equipment was built in order to evaluate the best possible continuous grinder loading principles, although this excluded the defiberation stage itself.

Based on these findings, another project has now been established to install an additional continuously operating grinder pocket into the existing one-pocket pressure grinder at Valmet Pulping Technology Center (VPTC). The new installation will be started in the spring of 1999 in order to evaluate this new continuous grinding principle further. The final project target is to engineer and build a mill scale continuous two-pocket grinder to be started in the beginning of the new millennium.